Down syndrome (Ds) is the most common chromosomal cause of intellectual disability that results from triplication of chromosome 21 genes. Persons with Ds demonstrate cognitive deficits in addition to co- morbidities that often accompany Ds, including cardiovascular abnormalities, thyroid disease, obesity, hypotonia and muscle weakness, upper airway obstructions, and sleep apnea. Although sleep apnea is a prevalent disorder in children and adults with Ds, the mechanisms responsible for these breathing deficits have not been elucidated. Our preliminary data reveal attenuated minute ventilation and mean inspiratory flow, and an increased number of apneas in Ts65Dn mice, a model of Ds; suggestive of ventilation deficits that may have a neural origin. Preliminary data also suggest impaired force production of diaphragm muscle from Ts65Dn mice in response to fatiguing muscle contractions. Together, these data suggest that the altered breathing patterns observed in Ts65Dn mice could be derived from neural and muscular origins. The experiments described in this proposal will further examine neural and diaphragm muscle contributions to breathing alterations in Ts65Dn mice and examine the activity of the proteasome, a major cellular proteolytic system, in the C3-C5 region of the spinal cord as a potential mechanism of breathing alterations. Aim #1 will characterize conscious ventilation in Ts65Dn mice. We will further characterize differences in the breathing pattern of Ts65Dn mice and reveal if arterial partial pressures of CO2 are elevated in conscious Ts65Dn mice vs. colony controls. Aim #2 will determine if breathing deficits of Ts65Dn mice are associated with reduced neural output of the phrenic nerve. These experiments will quantify the phrenic burst amplitude to measure the neural contribution to breathing. We will also assess proteasome activity of the phrenic nucleus to test the hypothesis that proteasome activity will be lower in Ts65Dn mice. Aim #3 will assess diaphragm muscle function in Ts65Dn mice. These experiments will determine if diaphragm from Ts65Dn mice exhibits increased susceptibility to fatigue in vitro compared to diaphragm from colony controls. All aims will study mice at 3 months, 12 months and 18 months of age to further describe aging in this model. Since individuals with Ds demonstrate characteristics of accelerated aging it will be important to know how ventilation changes with advancing age in Ts65Dn mice. Overall, this project will provide insight into the physiological systems that modulate breathing in Ds with the objective of improving the quality of life of persons with this condition. These experiments will support the sciences at Le Moyne College and engage undergraduate students in biomedical research to train the next generation of researchers.